The invention relates to catalyst compositions for use in the Fischer-Tropsch process for the conversion of synthesis gas into hydrocarbons, and an improved process using these catalyst compositions. It is particularly aimed at producing hydrocarbons suitable for use as liquid fuels.
The Fischer-Tropsch process is well known and described in various texts such as "The Fischer-Tropsch and Related Synthesis" by H. H. Storch, N. Golumbic and R. B. Anderson (John Wiley and Sons, New York, 1951). Generally this process takes place over metals such as iron, cobalt, nickel and ruthenium, which may be supported on carriers such as kieselguhr or silica. While nickel preferentially produces methane from synthesis gas, iron, cobalt and ruthenium produce a broad weight range of hydrocarbons ranging from methane to heavy waxes and consist principally of linear paraffins and olefins.
The hydrocarbons distributions of these products generally follow the Schulz-Flory distributions, which may be represented by the following equation: EQU W.sub.n +n.alpha..sup.n-1 (1-.alpha.).sup.2
where W is the weight fraction of the product with a carbon number n, and .alpha. (commonly known as the alpha value) is the probability of chain growth, and is assumed to be independent of chain length.
There is some deviation from this equation, especially at lower carbon numbers where independence of chain growth is less likely. Methane makes are generally "higher than expected", and other low carbon fractions are generally "lower than expected". This is believed to be caused by methane being formed by additional mechanisms such as cracking, and greater reactivity of lower olefins. (especially ethylene) towards chain growth.
As the products are predominantly straight chained in nature, the octane number of the gasoline fraction of the resulting liquid product is low. Thus in order to produce hydrocarbons suitable for use as liquid fuels, it is desirable to limit the chain length of the product to essentially the diesel range, and to produce a gasoline fraction of enhanced octane number by increasing levels of branching and/or aromatics in the product.
Zeolites are crystalline aluminosilicates with shape selective and acidic properties, and are further described in texts such as "Zeolite Molecular Sieves" by D. W. Breck (John Wiley and Sons, New York, 1974). They, and ZSM-5 in particular, have been found to be extremely effective in converting methanol into highly aromatic gasoline range hydrocarbon mixtures.
Two different approaches using zeolites as catalyst components have been tried in order to achieve the abovementioned desirable results.
The first approach consists of using a catalyst mixture comprising a methanol synthesis catalyst and a zeolite.
For example, AU 53272/79 (Shell International Research Maatschappi BV) describes the use of a mixture of two catalysts to produce an aromatic hydrocarbon mixture. The first of these catalysts is capable of converting a H.sub.2 /CO mixture into acrylic oxygen contained hydrocarbons, whilst the second is a crystalline silicate. A catalyst containing zinc together with chromium was said to be very suitable for use as the first catalyst.
However, results obtained using this approach have proved to be disappointing, as in order to obtain reasonable conversion, too much methane is formed. This is due to the considerably different favorable temperature, pressure regimes for the operation of the two different catalysts.
The second approach uses bi-functional catalyst systems in which an active Fischer-Tropsch metal component is mixed and/or incorporated into or onto a zeolite, e.g.:
(i) U.S. Pat. No. 4,086,262 (Mobil Oil Corporation) describes the use of zeolites such as ZSM-5 as supports for Fischer-Tropsch metals including iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium, to produce hydrocarbons from synthesis gas.
(ii) AU 34883/84 (Union Carbide Corp.) describes the use of catalyst compositions consisting of steam-stabilized Zeolite Y as a catalyst support for conventional Fischer-Tropsch metals such as iron or cobalt. These compositions enhanced branching and aromatization in the products, as well as the amount of product boiling in the liquid fuel range.
(iii) AU 88929/82 (U.S. Department of Energy) describes a catalyst composition of cobalt, promoted with thoria, on a ZSM-5 type zeolite support to produce high octane liquid hydrocarbon products that are in the gasoline boiling range, but contain branched aliphatic hydrocarbons rather than aromatics to impart high octane numbers.
The specifications of Australian Patent Nos. 559306, 562460, 566159, 567204 and 570126 disclose Fischer-Tropsch catalysts comprising cobalt and a promoter supported on silica and/or alumina. In each specification chromium is mentioned as a suitable promoter. The catalysts exemplified in these specifications have relatively high cobalt loadings. Furthermore there is no indication that incorporation of chromium into the catalyst system decreases methane selectivity and increases liquid hydrocarbon selectivity.
While cobalt catalysts have higher activities and selectivities to liquid range hydrocarbons, iron catalysts are currently preferred because of the excessive amounts of methane produced by cobalt catalysts. Cobalt catalysts can produce ten times the amount of methane predicted by the Schulz-Flory equation, whilst excess methane produced by iron catalysts is generally minimal.
Thus there is considerable incentive to reduce the amount of methane produced by cobalt Fischer-Tropsch catalysts in order to make these catalysts commercially viable. It is therefore an object of the invention to reduce methane selectivity and increase liquid range hydrocarbon selectively for cobalt Fischer-Tropsch catalysts, whilst maintaining high catalyst activity, and producing a high octane gasoline fraction.